1. Field of the Invention
This invention relates to compositions for making multilayer ceramic (MLC) substrates and dielectric materials, particularly, to compositions which can be sintered at low temperatures.
2. Description of the Related Art
Generally, electronic packages are used to carry IC chips, to build interconnections, to provide proper heat dissipating medium, and to protect silicon chips from moisture, chemicals or light. Highly reliable electronic packages for military, high performance computer, avionic, and telecommunication applications are usually made of ceramic materials. Because of their high strength, high rigidity, excellent electric insulation, low thermal expansion coefficient, high thermal conductivity and chemical stability, currently multilayer ceramic packages find wide spread applications in consumer electronic products. They can meet most of the material requirements in the development of compact, high performance and multi-functional electronic products. The low thermal expansion coefficients of ceramics enables large size IC chip to be attached to the multilayer board without adverse effect.
Generally, multilayer ceramic packages can be divided into two categories: (1) high temperature type, which is fabricated with high sintering temperature material such as Al.sub.2 O.sub.3, AlN, or BeO, and (2) low temperature type, which uses glass-ceramic as its major constituent. The latter type is gaining popularity, because its low dielectric constant can reduce signal propagation delay time and its low-firing temperature enables the usage of low melting point, high conductivity conductors (such as Cu, Ag, Au, etc.), thus preventing degradation of the signal strength.
Multilayer ceramic circuit boards are produced by two methods: (1) a multilayer thick film method; and (2) a high temperature co-fired system.
In multilayer thick film method, a sintered ceramic substrate board is first formed and a layer of circuitry is printed on top of it and sintered. Subsequently, a layer of insulative dielectric material is printed onto the circuit layer and then sintered. By repeating these printing and sintering steps, a multilayer ceramic circuit board is obtained. Such a process suffers from the following disadvantages: (1) many printing steps are required; (2) many sintering steps are required; (3) the thickness of the dielectric layers are difficult to control; and (4) the number of printing layers are limited. However, the advantages of such a process are as follows: (1) sintering temperature is low (850.degree. to 900.degree. C.); (2) low resistance conductive metals can be used (such as Ag, Au and Cu); and (3) investment costs are low.
In the high temperature co-fired system, Al.sub.2 O.sub.3 is the most popular material. It is first mixed with an organic binder and formed into green sheets. Electric circuitry is then formed by screen printing a metal paste on top of the green sheet. The green sheets are then laminated and sintered at an elevated temperature. The disadvantages of this process are: (1) the sintering temperature is high (1500.degree. to 1600.degree. C.); (2) it is necessary to use refractory metals (such as W, Mo) as conductor which have lower conductivity; (3) the dielectric constant of Al.sub.2 O.sub.3 is 9 to 10 which is relatively high and can result in signal propagation delay; and (4) investment costs are high. The advantages of this process are as follows: (1) the finished product is sintered in one time; (2) the thickness of the dielectric layer is easy to control; (3) the number of laminates is not limited; and (4) the surface roughness is small.
In recent years, a low temperature co-fired system which has all of the advantages of the abovementioned two methods has been developed for manufacturing multilayer ceramic circuit boards. The major constituents in this low temperature co-fired system are glass-ceramic materials. With these materials, the product can be made by a one-step sintering process and the sintering temperature is low (850.degree. to 950.degree. C.). Therefore, high-conductivity metals such as Ag, Ag-Pd, Au, and Cu can be used to construct the interconnections. In addition, such a low firing MLC board has low dielectric constant (4 to 8) and the investment cost is low.
The proposed ceramic materials system includes two categories: (1) crystallized glasses, and (2) glass + ceramics. Crystallized glass usually employs a crystallized glass containing .alpha.-cordierite or .beta.-spodumene as a major constituent for the insulating material and is disclosed in U.S. Pat. No. 4,301,324 and Japanese Patent No. Kokai 128856. Glass + ceramics uses an amorphous glass as the matrix and ceramic powders as fillers and is disclosed in many U.S. Patents, such as U.S. Pat. Nos. 4,953,006 and 4,547,625. The system with glass + ceramic is more popular because the physical properties of the substrate can be adjusted by adding various ceramic fillers and its cost is relatively low. Nevertheless, there is still a strong desire to provide an improved composition for this low firing temperature multilayer ceramic substrate and dielectric material with low resistance conductors, low dielectric constant, high sintered density, high mechanical strength and that meets the industrial requirements of thermal expansion coefficient for a of IC elements.